"HEARING" the Lower Flutes

Playing the Low Flutes can truly be a challenge! These instruments have the same fingerings, and even look similar (for the most part) to the C Flute, but they are certainly tubes of a different nature. Players need to slightly reshape the embouchure (relax the opening of the lips, and allow a bit of the upper lip to hang over the tone hole), allowing a more open volume of air to pass through the lips, yet remain slow in air speed. Another technique is to allow the lower lip to be placed partly in the tone hole itself, of course this will assist with changing the angle of the air stream, and may help with bringing a bit of edge to the low tones.
In my opinion, the greatest challenge is not the physical aspect of playing, yet this can be most demanding for some, but rather aural (hearing). One needs to reshape their hearing when playing the larger flutes, and understand that the richest pitches are from another world, the 'bass clef!' A low C on the Bass flute, only one octave lower than the C Flute, is in actuality, quite a different quality and tone color, just by basic laws of physics- it resonates at a slower rate, it has a wider frequency band and is clearly filled with all sorts of harmonic partials! Embrace that sound and amplify it as much as you can. Just look at the tubing of the bass or Contrabass flutes, your tiny airstream from your pursed lips, has a long (and wide) way to travel.
Sometimes I hear doublers on the lower instruments, playing with a closed off, hollow sound. They may even struggle to find those low pearls on the alto or bass. Allowing the ears to realize that these lower instruments are transposing, and only with the use of the ears and acknowledgement of pitch placemnet, will one find the core sound of the lower flutes. This of course takes time, and I myself, am still learning this, after years of playing bass flute in a professional flute quartet. Playing the cello part of a Mozart string quartet is no simple feat, trust me!
So a good concept to formulate is: Train the ears to accept and work in these lower frequencies.
Practice singing in bass clef. Even ladies can do this, as I can sing a soprano high C, as it is my ear that allows the sound to come out, not my brilliant vocal technique.
One will find that this key concept of aural understanding will lead to a sound and color that is most desireable on any of the low flutes (Alto, Bass, or Contra Bass).